Deep inspiration while the nurse/doctor inflates the cuff, and holding your breath.

There is a physiologic response to deep inspiration (breathing in) that lowers your systolic blood pressure by about 10 mm Hg. That only lasts during inspiration, but if you do it at the right moment, it's all you need to “cheat” on your BP test.

During inspiration the pressure in your chest (thoracic) cavity becomes negative relative to atmospheric pressure. That happens simply because the diaphragm contracts and moves downward, expanding the chest cavity and increasing the volume of the thoracic cavity. This creates a more negative intrathoracic pressure (the pressure within the chest cavity).

With lower pressure, the volume of the vena cava increases, and therefore more blood can be delivered to the right chamber. Since the right and the left chamber share a wall between them (the septum) this wall tends to bulge a little into the left chamber, thereby reducing the amount of blood that the left chamber can accommodate. This effect of lesser blood in the left chamber is augmented by concomitant pooling of the blood in the lung's (pulmonary) vasculature that drains into the left chamber. The net effect of all this - increased venous return to the right side of the heart, decreased volume of the left chamber and decreased return from the pulmonary vasculature - is an approximate drop in systolic pressure by 10 mmHg. So, when the nurse starts inflating the cuff, you engage this mechanism by inhaling deeply and holding your breath while she slowly deflates the cuff to measure your blood pressure.